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Astron. Astrophys. 352, L87-L90 (1999)

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1. Origin of the black hole binary runaway velocities

There are two effects to accelerate a binary system by a supernova explosion. The first is caused by the ejection of material from the binary (Blaauw 1961). The centre of mass of the ejected matter will continue to move with the orbital velocity of the black hole progenitor. To conserve momentum, the binary will move in the opposite direction. The second one is an additional velocity kick, which is produced by asymmetries in the supernova explosion itself and for which there is strong evidence in the case of the formation of a neutron star (e.g. Lyne & Lorimer Lyne and Lorimer (1994), Hartman Hartman (1997)).

The current status quo of supernova simulations is that in order to get a successful supernova, in which the shock is reversed and matter is ejected, one needs to form a neutron star Bethe and Wilson 1985. If the supernova is not so energetic, there may be considerable fall back, turning the neutron star into a black hole (e.g. Colgate Colgate (1971); Woosley & Weaver Woosley and Weaver (1995)). Formation of a black hole without an intermediate neutron star would then not result in mass ejection. However, if other mechanisms than neutrino heating will be found to reverse the supernova shock (e.g rotation), this conjecture of both mechanisms may be broken.

Brandt et al. Brandt et al. (1995) have listed a number of scenarios for reproducing the high radial velocity measured in Nova Sco. They show that though mass ejection alone can explain the velocity of Nova Sco 1994, the allowed range of initial masses is very small. They therefore conclude that Nova Sco is formed in a delayed black hole formation, in which the kick, which is imparted to the initial neutron star, is responsible for a considerable fraction of the present system velocity. The black holes in the other binaries would then be formed by a direct collapse without mass ejection and kicks. The velocity dispersion found by White & van Paradijs White and van Paradijs (1996) can be explained by scattering at molecular clouds and density waves, since these binaries could be an old population (Podsiadlowski, private communication; see also Brandt et al. Brandt et al. (1995)).

With the new discovery of the relatively high velocity of Cyg X-1, we think the above is unlikely, because now the two systems with highest mass companions must have formed through a delayed black hole formation, while the systems with low mass companions form in a direct collapse. This would mean that the success of the SN in which the black hole is formed is related to the nature of its binary companion, for which we see no reason

Tutukov & Cherepahshchuk Tutukov and Cherepashchuk (1997) discuss the system velocities of the X-ray binaries containing black holes and conclude that all velocities can be explained with mass ejection alone. However, they only consider the maximum velocity that can be obtained with the observed limits on the masses of both stars, assuming the shortest possible period at the moment of the SN and the maximum amount of mass that can be ejected without disrupting the binary. In that case, the pre-SN mass ratio is not independent of the final (observed) mass ratio and it would be better to use the current (observed) mass ratio, with which their Eq. (7) would become

[EQUATION]

See also the discussion in Sect. 4.

We now investigate the effect of the mass ejection in more detail, assuming possible kicks are (relatively) unimportant.

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999
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